Brianna from the Google Maps team announced in a thread that they have released a brand new help document on how to handle these situations.

You can find the help document over here. It breaks the duplicate issues into two categories.

(1) Do multiple results for your business appear in Google Maps search results?
(2) Are there multiple listings in your Google Places account for the same business location?

The answers on what you should do depend on the above.

(1) Do multiple results for your business appear in Google Maps search results?

Make sure you've claimed one of the listings in Google Places. Verify that there’s only one active listing in your account by logging in to your Google Places account.

If there's more than one listing in your account for the same business, you’ll first need to fix that problem by following the steps in the section below.

If you don't have a listing for it at all, you should claim your business.
For every extra result for your business, go to its Place Page, click the Report A Problem link, and select "Place has another listing". If you can, include a link to your claimed place page in the comments section.

Google will review these reports and fix duplicate business listings.

(2) Are there multiple listings in your Google Places account for the same business location?

Choose the listing that you'd prefer to keep in your account.

Make sure that you have all your enhanced content (photos, business hours, description) attached to this listing.

Make sure that its status is "active".

Delete all other listings for the same business from your account. Choose the "Remove this listing from my Google Places account" option

These changes can take several weeks to appear in search results

If there are multiple results for your business in Google’s search results, follow the instructions above.